PROJECT SUMMARY: Complement component C3 is important for removing pathogens and plays a role in synaptic refinement during brain development. Complement accrues in Alzheimer's disease (AD) plaques (Stoltzner et al., Am J Pathol 2000). We have found that C3 and its downstream signaling fragments play important roles in synapse loss during normal aging in hippocampal CA3 of WT mice (Shi et al., J Neurosci 2015) and in the response of glia to A? and subsequent synaptic and neuronal changes in AD mice (Shi et al., Sci Transl Med 2017). We have shown that microglial uptake and clearance of A? is at partially mediated by the interaction of C3b with its receptor, CR3, on the surface of microglia (Fu et al., GLIA 2012). Together with our collaborators, Dr. Hong and Stevens, we recently reported that complement C1q and C3 mediate very early synapse loss induced by A? oligomers in the absence of A? plaques in mice (Hong et al., Science 2016). We further reported that lifelong, germline C3 knockout (C3KO) was neuroprotective in aged APP/PS1dE9 mice, despite increasing the plaque load (Shi et al., Sci Transl Med 2017). C3-deficiency in APP/PS1dE9 mice dampened the response to A? plaques, which correlated with normalization of cognition to that of WT mice. However, it remains unknown if C3 suppression after the onset of AD pathogenesis will be protective and the cell type by which this effect is mediated. Therefore, we generated C3 floxed mice and used them to generate a novel inducible C3 conditional KO model (UBC-Cre-ERT2;C3fl/fl), named ?C3iKO?, in which tamoxifen treatment at any age induces global C3 deletion. We hypothesize that C3-deletion in early stages of AD pathogenesis will protect the hippocampus against downstream neurodegeneration and, that this effect is cell- specific for microglia. Here, we propose the following Aims: 1. Determine whether induction of global C3- deletion shortly after the onset of plaque deposition in APP/PS1dE9 mice (6 mo) protects against late hippocampal changes (16 mo) in female and male mice. 2. Determine whether induction of global C3-deletion shortly after the onset of tau pathology in P301S;hApoE4TR (TE4) mice (2 mo) protects against late hippocampal changes (9 mo) in female and male mice, and if C3 effects are related to ApoE4 signaling. 3. Cross our unique C3 floxed mice to generate 2 novel, cell-specific inducible C3 cKO mouse models in which tamoxifen treatment induces C3 deletion in either microglia or astrocytes (2 mo) and determine the functional response (hippocampal LTP) to aging, fibrillar A?, tau aggregates and human ApoE4 (4 and 12 mo). Microglial and astrocyte gene expression will be assessed in all Aims to better understand signaling mechanisms in C3- mediated neurodegeneration. The overall goal of our study is to determine whether C3-deficiency shortly after the onset of AD pathogenesis is protective against neurodegeneration and cognitive decline, and whether such protection is mediated by microglia or astrocytes. These studies may guide complement-lowering therapies for AD and other neurodegenerative diseases and will provide novel research models to many fields.